The Fischer Tropsch process can be used for the conversion of hydrocarbonaceous feed stocks into liquid and/or solid hydrocarbons. The feed stock (e.g. natural gas, associated gas and/or coal-bed methane, residual oil fractions, biomass and coal) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas). The synthesis gas is then fed into a reactor where it is converted over a suitable catalyst at elevated temperature and pressure into paraffinic compounds ranging from methane to high molecular weight modules comprising up to 200 carbon atoms, or, under particular circumstances, even more.
Catalysts used in the Fischer-Tropsch synthesis often comprise a refractory oxide based support material, especially titania, and one or more metals from Group VIII of the Periodic Table, especially from the iron group, optionally in combination with one or more metal oxides an/or metals as promoters. Particular interest has been given to catalysts comprising cobalt as the catalytically active component, in combination with one or more promoters selected from zirconium, rhenium, chromium, vanadium, platinum and manganese, especially manganese. Such catalysts are known in the art and have been described for example, in the specifications of International Patent Application No. WO A-9700231 and United States Patent publication No. U.S. Pat. No. 4,595,703.
Catalysts can be prepared by obtaining a metal hydroxide, carefully oxidising it to the metal oxide and then placing it in the appropriate reactor where it is reduced to the metal in situ.
One catalyst for Fischer-Tropsch reactions is cobalt on titania. To prepare the catalyst, cobalt hydroxide (Co(OH)2) can be used as a starting material. This material is mixed with the carrier, dried, calcined and then decomposed to form cobalt oxide (CoO). The cobalt is further oxidised (Co3O4) and then placed in a Fischer-Tropsch reactor. In the reactor the cobalt oxide is reduced to cobalt.
It is known to use a “peptising agent” in the preparation of catalysts or catalyst precursors. The peptising agent may dissolve small amounts of the constituents and may separate any agglomerated crystals. Agglomerated crystals include any loosely bonded crystals, but do not include any highly bonded crystals, grown crystals or single crystals.
It is preferred not to add any excess of the peptising agent because too much organic material present in the sample during calcination can cause problems and can lead to uncontrolled partial reduction of cobalt hydroxide to cobalt titanate, thus reducing the activity of the catalyst. Thus it has generally been regarded that the maximum amount of peptising agent that should be added to a catalyst starting material, such as cobalt hydroxide, is 0.75 wt %.